1. The Field of the Invention
This invention relates to a method and system for transmitting high frequency communication signals over a preexisting building power line network. More particularly, the present invention relates to a method and system for using anti-resonance isolation and virtual earth ground signaling to transmit communication signals over wires of a power line network.
2. The Relevant Technology
The ability to freely access data on a network and to transfer information between electrical apparatus can dramatically increase productivity and efficiency. Networking is beneficial for businesses, as well as for residential uses. Transfer of data over a network is typically accomplished with a telephone line or cable. Accordingly, many buildings are wired for local access network (LAN) connectivity. Some buildings, however, particularly older residential, business, and military buildings, are not appropriately wired for LAN. These buildings must either be rewired, or an alternative means for networking must be used. Because the costs of rewiring a building can be prohibitive, it is desirable to provide an efficient means of networking over preexisting building wires. One such means includes the transfer of communication signals over preexisting power transmission lines.
The use of power transmission lines as a communication network is well known in the art. Early inventors contemplated sending communication signals directly over the load carrying conductors of the power line. There are problems, however, associated with transmitting communication signals over load carrying conductors (wires). Perhaps the most significant problem, is xe2x80x98noise,xe2x80x99 which is resident in all power line networks.
Noise is generated in the load carrying conductors by the high voltage alternating power, the discontinuities in impedances caused by various branch circuits, transients and impedance changes produced by power load switching, and isolation of the power line network into separate circuits. Noise makes it difficult to transmit high frequency signals over long distances due to line losses, radiation, and impedance mismatches.
Noise also interferes with the transmission of communication signals by limiting the capacity of the network to transmit reliable communication signals. The constraining effect of noise on data transmission is defined by the Hartley-Shannon Law, in which C=B*log2(1+P/N)bits/s. In this equation, C establishes the upper limit for the rate of reliable information that can be transmitted over the conductor, B is the bandwidth, P is the average power of the transmitted signal, and N is the average power of the noise component. Accordingly, as the noise in the conductor increases, the capacity of the conductor to transmit reliable data decreases.
Various approaches have been proposed to overcome the constraints of noise and to enable reliable transmission of communication signals over power line networks. For example, in one approach, which is disclosed in U.S. Pat. No. 4,697,166, issued to Warnagiris et al., selective filters are used to separate power signals and communication signals at the transmitting and receiving ports of a communication system on a power line network. In another approach, which is disclosed in U.S. Pat. No. 4,864,589, issued to Endo, different transmission frequencies (frequency hopping) and xe2x80x9cspread spectrumxe2x80x9d systems are utilized to transmit communication signals over a power line network. In yet another approach, as disclosed in U.S. Pat. No. 5,982,276, issued to Stewart, electromagnetic signals are transmitted through magnetic modulation of the magnetic flux surrounding the load carrying wires. These various approaches disclose various means of transmitting communication signals over the main current carrying conductors of a power line network. They do not, however, directly address or overcome the underlying problems associated with noise, namely, limited bandwidth and slow transfer rates. Rather they simply enable the transmission of communication signals over the resident noise.
To effectively overcome slow transfer rates and limited bandwidth of noisy channels, communication signals may be transmitted over conductors with less noise, such as between the earth ground and the power line neutral, instead of using the xe2x80x9chotxe2x80x9d power line, as disclosed in the prior art. By transferring communication signals between the earth ground and the power line neutral, the effects of noise are minimized because these conductors have less noise. One problem with this approach, however, is that to transmit signals between the building ground and neutral power lines, it is necessary that the two conductors be electrically isolated, which they are not.
In the prior art, as disclosed in U.S. Pat. No. 3,702,460, issued to Blose, adequate isolation between the building ground and neutral lines is accomplished by placing a transformer winding between the two conductors at the transmitting end and at the receiving end of the power line network. Although this resolves many of the problems associated with noise when using the current carrying wires of the power line network, this method is not appropriate for buildings that are wired to current electrical codes. In present residential and commercial buildings, for example, the uniform electrical code requires that the building ground, earth ground and the power line neutral be conductively tied into a common heavy bus or xe2x80x9ctiexe2x80x9d at the service panel. This xe2x80x9ctiexe2x80x9d is commonly referred to as a ground bar.
In U.S. Pat. No. 4,433,326, issued to Howell, an alternative method of isolating the building ground from the neutral line is proposed, which involves replacing the xe2x80x9ctiexe2x80x9d at the service panel with an inductor or transformer. This modification is intended to isolate the ground and neutral lines at high frequencies for transmitting communication signals while at the same time enabling a tie between the conductors at lower power line frequencies, so as to satisfy the safety codes. This, however, requires modification to the building wiring at the service panel and generally requires the services of a licensed electrician and the use of a special approved inductor, which can be costly. This prior art also does not resolve problems of parasitic capacitive coupling between the neutral and building ground wires.
Furthermore, by replacing the xe2x80x9ctiexe2x80x9d with an inductor or transformer, the problems associated with having to rewire a building to enable network connectivity are not resolved. In particular, costly professional rewiring of the power line network is still required. Accordingly, it would be desirable to provide a method and system for networking over preexisting power lines without requiring any electrical modifications to the preexisting power lines or service panel.
In accordance with the invention as embodied and broadly described herein, a system and method for sending and receiving high-frequency signals over a previously installed building power line network is provided. A suitable environment for practicing the present invention is a previously installed building power line network that includes an electrical service panel, a building ground line, a hot line, and a neutral/earth ground line. The service panel connects a utility phase I power line, a utility neutral power line, and a utility 115V phase II power line to preexisting building wires that include the hot line, the neutral/earth ground line and the building ground line. To comply with electrical safety standards, the service panel also includes a ground bar that provides a common conductive xe2x80x98tiexe2x80x99 for various ground and neutral wires, including an earth ground wire that connects the ground bar to the physical ground (earth).
The building wires are insulated and routed through cabling to various power outlets throughout the building. The power outlets include at least one set of three electrical contacts that are correspondingly connected to the hot, neutral and ground power lines. Each of the electrical contacts is configured to receive and electrically couple with a corresponding prong of a standard three-prong house electrical plug.
The power line communication system comprises a transmitter and a receiver. The transmitter includes a high-frequency transmitter, and at least two tuning elements, or devices. As used herein, the terms xe2x80x9ctuning elementsxe2x80x9d and xe2x80x9ctuning devicesxe2x80x9d are interchangeable. One tuning element is connected between a xe2x80x9chotxe2x80x9d wire and a neutral wire of the transmitter and the other tuning element is connected between the neutral wire and a ground wire of the transmitter. The two tuning elements and the high-frequency transmitter are all encased within a transmitter chassis.
Two additional tuning elements are also connected to the transmitter. One of the additional tuning elements is connected between the building ground line and the chassis of the transmitter. The other additional tuning element is connected between the neutral line and the chassis of the transmitter.
The receiver includes a high-frequency receiver enclosed in a chassis along with two tuning elements. Two additional tuning elements are also connected to the receiver. One of the additional tuning elements is connected between the building ground and the chassis of the receiver. The other is connected between the neutral line and the chassis of the receiver.
The receiver and the transmitter are electrically coupled with the hot, building ground, and neutral wires, of the power line network, at remote outlet receptacles with conventional three-prong house electrical plugs. The receiver and transmitter may be located on different circuits of the power line network and may even be connected to different phases of the xe2x80x9chotxe2x80x9d power. The transmitter and receiver can be located on any outlet or circuit, of the power line network, so long as they share a common service panel and ground xe2x80x9ctie.xe2x80x9d
The positions of the transmitter and the receiver on the power lines may be reversed to allow the possibility of using switches or analog multiplexing circuits to reverse the transmit and receive functions of the transmitter and receiver, and to achieve half or full duplex communication, depending upon the type of multiplexing circuits used.
The tuning elements comprise one-port networks that include electronic circuitry for performing desired tuning. The electronic circuitry may include passive capacitors, inductors, and/or resistors, arranged to provide the desired tuning function for each of the tuning elements. The desired tuning may also be implemented with active filter components, which may employ operational amplifiers, gyrators or other active devices along with passive components.
The high-frequency transmitter generates a sinusoidal carrier signal that is modulated in accordance with an input signal. The type of modulation used may include amplitude modulation, frequency modulation, frequency shift keying, phase modulation, quadrature amplitude modulation or any other method of modulating a carrier signal to encode either analog or digital signals on the carrier. The carrier signal is impressed between the neutral and building ground wires at the power outlet where the high-frequency transmitter is installed. Even though the building ground and neutral wires are conductively tied to a common ground bar at the service panel, they will not appear as a short circuit to the high-frequency transmitter, at the power outlet, because the inductance of the electrical wires as well as the wire-to-wire capacitance is significant at high-frequencies and effectively isolates the conductors at high-frequencies. Depending on the particular tuning elements chosen for the transmitter and the receiver, the building ground and neutral wire may exhibit a transmission line effect or an antenna effect when the carrier signal is impressed between them.
The carrier signal is chosen to be at a frequency that creates a parallel resonance (anti-resonance) between the building ground and earth ground/neutral lines. This electrically isolates the building ground from the neutral line. The tuning elements, that were previously identified, further isolate the building ground from the neutral line. This anti-resonant isolation effectively provides a means for transmitting communication signals over low noise power conductors, without requiring transformer windings or other modifying hardware to isolate the earth ground line and the building ground line.
In the foregoing manner, the power line network of an existing building can be used to send and receive high-frequency signals without modifying the power line network. This feature is a significant improvement over conventional techniques that require an existing power line network to be modified or physically retrofitted to be used to transmit network data in the building. Moreover, the methods of the invention overcome high voltage noise, frequency and bandwidth limitations, impedance mismatches, and other reliability problems associated with many conventional approaches to transmitting data over power line networks. In addition, the invention can be practiced in ways that comply with generally accepted building codes.